1. Field of the Invention
The present invention relates generally to fluoride-containing bioactive glass compositions of highly specific compositional ranges having substantially improved resistance to demineralization which also demonstrate the capability of forming an extensive layer of crystallized mixed apatite (carbonate-fluorohydroxylapatite) phases when exposed to physiological solutions. The fluoride-containing bioactive glass compositions of the instant invention are useful as prosthetic implants or implant coatings in dental and surgical applications where optimization of durable chemical bonding with living tissue is desirable.
2. Brief Description of the Prior Art
Bioglass.TM. 45S5 is a bioactive glass containing 46.1% SiO.sub.2, 2.6% P.sub.2 O.sub.5, 26.9% CaO and 24.4% Na.sub.2 O, all percents being mole percents, and was developed to promote the formation of direct chemical bonding at the interface between mammalian bone and the glass, the direct bond obtained without the requirement for a porous structure. The advantages of the nonporous structure are, among others, that the intrinsic high strength of the material is retained and that the glass is useful as a coating for high strength materials such as stainless steel or aluminum oxide.
In an article entitled "Direct Chemical Bonding Between Bioactive Glass-Ceramic Materials and Bone" by Hench et al, Journal of Biomedical Materials Research Symposium, No. 4 (1973), pages 25-42, experimental work was reported on a single glass composition wherein 50 molar % of the CaO of the 45S5 glass was replaced with CaF.sub.2. This bioactive glass containing 46.1% SiO.sub.2, 2.6% P.sub.2 O.sub.5, 13.45% CaO, 13.45% CaF.sub.2 and 24.4% Na.sub.2 O, all percents being mole percents, is designated Bioglass.TM. 45S5F and initially seemed to demonstrate increased resistance to aqueous attack and increased rate of apatite film formation as compared to the non-fluoride containing bioactive glass formulation.
However, in-vivo bone bonding studies of the 45S5F glass in the mid femur of mature rats indicated that bone bonding did not occur. This work is reported in a Greenlee, Jr. et al article published in the Journal of Biomedical Materials Research, 6 (1972), pages 235-244. Based on this work, it was concluded that the fluoride-containing bioactive glass has a much lower surface reactivity than the non-fluoride containing counterpart, this decreased surface reactivity making the glass unsuited for many implant applications.
A companion intramuscular implant study, the results of which were reported in the Hench et al article supra, pages 32-36, indicated that the cellular response to the 45S5F glass is fibroblastic in nature with the formation of elongated fibroblasts and large intercellular mature collagen fibers. Additionally, adjacent to the implant there was formed a row of large ovoid cells with single nuclei and foamy cytoplasm which were tightly adhered to the implant. The implant was stable and there was no adverse tissue reaction.
Stanley et al, Oral Surgery, Oral Medicine, Oral Pathology, 42(3) (1976), pages 339-356, "The Implantation of Natural Tooth Form Bioglass in Baboons", showed that the 45S5F glass used as dental implants in-vivo in baboon demonstrated a bonding ability equal to, but no better than, the 45S5 glass, with histopathological findings suggesting certain advantages residing in the 45S5F glass as compared to the 45S5 glass.
U.S. Pat. Nos. 4,159,358 and 4,234,972 to Hench & Buscemi disclose the 45S5 and 45S5F bioactive glass as bioactive coatings for prostheses comprising a metal substrate. Only the specific 45S5F bioactive glass containing 13.45 mole percent calcium fluoride is disclosed, the patents containing no suggestion that other fluoride contents might be useful. And while the references teach the 45S5F glass to be "extremely strong and resistant to body fluids", the references equate the 45S5 and 45S5F glasses.
U.S. Pat. No. 4,189,325 to Barrett et al discloses a large number of glass-ceramics for use in dental restoration work. The disclosed and claimed glasses contain 25 to 33 mole percent of Li.sub.2 O.sub.3, 52 to 73.5 mole percent SiO.sub.2, 0.5 to 5 mole percent Al.sub.2 O.sub.3 and 1 to 10 mole percent CaO. However, the patent contains no reference to calcium fluoride as an additive and nothing to lead one with ordinary skill to make such a substitution.
U.S. Pat. No. 4,171,544 to Hench et al discloses compositions having, or capable of developing, a high specific area, porous, silica-rich surface capable of forming strong bonds with bone tissue. Highly porous glass and glass-ceramic compositions are disclosed, said compositions containing at least 80 weight percent SiO.sub.2, hardened inorganic cements, and known biologically active glasses and glass ceramics. The high area silica-rich surface was thought to provide a large number of reactive sites for deposition and interaction of various of the organic and inorganic components of healing bone. However, the highly porous nature of the glass has a practical lack of utility because it cannot develop the requisite strength.
Thus a need has continued to exist for bioactive glass compositions which have sufficient resistance to demineralization in physiological fluids and sufficiently high proclivity for and rate of formation of mixed apatites in the reactive surface layer to provide a highly durable implant/tissue chemical bond to form with living tissue.